An example of such facsimile communication system as described above is shown in a schematic block diagram of FIG. 1. Since the communication system shown in FIG. 1 is the same as an outlined system configuration to which the present invention is to be applied, the prior art will be described hereinafter by referring to FIG. 1.
In FIG. 1, facsimile terminals 1, 2, 3, and 4 are those standardized in accordance with ITU-T Recommendations T.4 and T.30, and these facsimile terminals conduct facsimile communications with each other through mobile terminals 5, 6, and 7, a multiplexed radio link, mobile communication control units 8, and 9, and a public telephone network 10.
The mobile terminal 5 functions to connect the facsimile terminal 1 to the mobile communication control unit 8. The mobile terminal 5 transmits to and receives from the facsimile terminal 1 transmission control procedure signal as well as image signal of analog signal. Furthermore, the mobile terminal 5 converts these analog signals transmitted from the facsimile terminal 1 into radio signals as a result of encoding digitally these signals, and transmits these encoded signals to the mobile communication control unit 8 through the multiplexed radio link. On one hand, the mobile terminal 5 receives facsimile transmission control procedure signal and image signal of radio signal, which have been digitally encoded and transmitted from the mobile communication control unit 8. These signals received are converted into analog signals to be transmitted to the facsimile terminal 1 in the mobile terminal 5. The same operations as that described above are implemented also in the mobile terminals 6 and 7, respectively.
The mobile communication control unit 8 is connected with the facsimile terminal 4 and the mobile communication control unit 9 through the public telephone network 10. The mobile communication control unit 8 implements a digital facsimile communication via a multiplexed radio link, while it makes an analog facsimile communication with the facsimile terminal 4 through the public telephone network 10. The mobile communication control unit 9 conducts also the same operations as that described above in the mobile communication control unit 8.
In the mobile communication control unit 8, radio communication equipment 81 receives facsimile transmission control procedure signals and image signals, which have been digitally encoded, of radio signals transmitted from the mobile terminal 5 through a multiplexed radio link, and transmits these signals to a communication control device 82. Furthermore, the radio communication equipment 81 receives facsimile transmission control procedure signals and image signals, which are transmitted from the communication control device 82 and have been digitally encoded, and converts these signals into radio signals to transmits to the mobile terminal 5 via a multiplexed radio link.
The communication control device 82 is connected with the facsimile terminal 4 through a trunk circuit 84 and the public telephone network 10, and transmits to and receives from the facsimile terminal 4 facsimile transmission control procedure signals and image signals of analog signals. Moreover, the communication control device 82 receives facsimile transmission control procedure signals and image signals of analog signal, which are transmitted from the facsimile terminal 4, and converts these signals into digitally encoded signals to deliver the encoded signals to the radio communication unit 81. Besides, the communication control device 81 converts facsimile transmission control procedure signals and image signals, which are delivered from the radio communication equipment 81 and have been digitally encoded into analog signals defined by the Recommendations T.4 and T.30 to transmit these analog signals to the facsimile terminal 4.
There are three types of connection mode in which facsimile terminals are connected with each other. A first connection mode is the one wherein the facsimile terminal 1 is connected with the facsimile terminal 4 through the mobile terminal 5, the mobile communication control unit 8, and the public telephone network 10. A second connection mode is the one wherein the facsimile terminal 1 is connected with the facsimile terminal 3 through the mobile terminals 5 and 7, the mobile communication control units 8 and 9, and the public telephone network 10. A third connection mode is the one wherein the facsimile terminal 1 is connected with the facsimile terminal 2 through the mobile terminals 5 and 6, the mobile communication control unit 8, and the public telephone network 10.
In case of the third connection mode, the facsimile terminals 1 and 2 are connected with the communication control units 82 and 83, respectively, and the trunk circuits 84 and 85 are connected with each other via the public telephone network 10. Such turn-back connecting mode of trunk line involves two types of manner, i.e., one of them is a case wherein the trunk line is turned back in a separate network (the public telephone network 10 in this example) outside the mobile communication control unit 8, and the other is a case wherein the trunk line is turned back in the mobile communication control unit 8 as shown in FIG. 1.
In the first connection mode, a facsimile transmission control procedure between the communication control device 82 and the facsimile terminal 4 must be based on the Recommendation T.30. In the second and the third connection modes, there is no recommendation defined specifically as a standard relating to a facsimile transmission control procedure for relay use passing through a trunk circuit, but a procedure based on the Recommendation T.30 is usually employed. In such connection modes, however, there is a delay derived from influences due to erroneous redelivery control and the like between a transmission and its response in a mobile communication control unit including a radio link. Accordingly, when the procedure based on the Recommendation T.30 is used with no modification, there is a possibility of a breakdown in communication.
The present invention provides a facsimile transmission control procedure for relay use for solving the problem described above, and it relates to a facsimile transmission control procedure for relay use passing through a trunk line in the second and the third connection modes.
In the following, a facsimile transmission control procedure for relay use in the above-described second and third connection modes in a conventional mobile communication control unit will be described by referring to FIG. 8. FIG. 8 is a diagram showing a communication sequence in the case where a procedure based on the Recommendation T.30 is used as the one for facsimile transmission control for relay use between mobile communication control units.
After connecting a communication line via mobile communication control units 8 and 9 including a radio link, and a public telephone network 10 between a transmit terminal (a facsimile terminal 1 in this case) and a receive terminal (a facsimile terminal 3 in this case), the transmit terminal delivers a calling signal CNG (hereinafter referred to as “CNG tone”), while the receive terminal transmits a called subscriber identification signal CED (hereinafter referred to as “CED tone”).
Such CNG tone and CED tone are relayed in the mobile communication control units 8 and 9, and they are transmitted to the receive terminal and the transmit terminal, respectively. These tones are not relayed inside the mobile communication control units 8 and 9 as they are, but they are relayed in both the units 8 and 9 after they were converted into digital signals, which indicate that these tones were detected by the communication control devices 82 and 92 in both the units. The receive terminal delivers NSF, CSI, and DIS constituted into frames, respectively, following to the CED tone.
It is to be noted that these NSF, CSI, and DIS are signals defined by the above-described Recommendation, and these signals in addition to the other names of signals are indicated in FIG. 13 by their full spellings and facilities.
The term DIS means a signal for notifying all the facilities, i.e., a facsimile group number, transmission and reception facilities, a terminal constant and the like involved in a receive terminal to a transmit terminal. It is to be further noted that operations of from line connection to reception of DIS by a transmit terminal must be completed within a period of time T1. The transmit terminal transmits NSS, TSI, and DCS to a side of the receive terminal after receiving the NSF, the CSI, and the DIS, and appoints a facility selected from among facilities indicated by DIS by the use of DCS. In this case, operations of from line connection to reception of DCS by a receive terminal must be completed within the period of time T1.
The transmit terminal transmits a training check signal TCF after transmitting DSC, and checks whether or not the other facsimile terminal by which an image signal (facsimile message signal) is to be received can communicate at a transmission rate selected by DCS.
The mobile communication control unit 8 is not allowed to pass TCF through its system, but does relay only NSS, TSI, and DCS. The mobile communication control unit 9 produces TCF to transmit it to a receive terminal after transmitting DCS to the receive terminal. When the receive terminal receives normally TCF and completes a preparation for receiving a message, it delivers CFR being a response signal representing to the effect that a delivery of message may be started.
The mobile communication control unit 8 transmits CFR to the transmit terminal in the case when the mobile communication control unit 8 receives normally the TCF transmitted from the transmit terminal and receives the CFR from the receive terminal. In this case, it is to be noted that operations of from transmission of the TCF to reception of the CFR in the transmit terminal must be completed within a period of time T4. When the transmit terminal does not receive any CFR within the time T4, it delivers again NSS, TSI, and DCS. The number of times for redelivery of CFR is fixed up to two, and when the transmit terminal does not receive CFR within the time T4 after a second time redelivery of CFR, in other words, a third time transmission of CFR, the transmit terminal transmits DCN to stop the communication.
The transmit terminal transmits an image signal based on the Recommendation T.4 within a period of time T2 after receiving CFR. When completing transmission of the image signal, the transmit terminal delivers EOP and notifies a completion of a page of the image signal as well as a fact to the effect that there is no other documents to be transmitted. The receive terminal waits EOP for the time T2 after receiving the image signal, and disconnects the line in the case where no EOP is received within the time T2.
When the receive terminal receives EOP within the time T2, it delivers to the transmit terminal MCF representing a fact that the image signal has been completely received in an acknowledgement of the EOP. It is to be noted that operations of from transmission of EOP to reception of MCF in the transmit terminal must be completed within the time T4. The transmit terminal redelivers EOP in the case when it receives no MCF within the time T4. The number of times for redelivery of EOP is fixed up to two, and when the transmit terminal does not receive MCF within the time T4 after a second time redelivery of MCF, in other words, a third time transmission of MCF, the transmit terminal transmits DCN to stop the communication.
When the transmit terminal receives MCF, it transmits DCN to the receive terminal to disconnect the line. Operations of from transmission of MCF to reception of DCN by the receive terminal are effected within the time T2.
As is apparent from the above, a strict temporal restriction from transmission of a command signal to reception of a response signal (T1 through T4) is requested in a facsimile communication in accordance with a transmission control procedure based on the Recommendation T.30, and at the same time, when it acts against such temporal restriction, a command signal is retransmitted, whereby it coops with communication troubles such as disappearance of a procedure signal due to data errors arising on a transmission path or an outage of line.
FIG. 9 is a diagram showing a format of a procedure control signal represented by binary code signals. In FIG. 9, although NSF, CSI, and DIS have been shown, but the other procedure control signals are the same as the former procedure control signals described above. The format shown in FIG. 9 is composed of preambles and control signals represented by binary code. In FIG. 9, the binary code is composed of respective frames of NSF, CSI, and DIS. The DIS frame contains a flag sequence (F) indicating a start and a completion of the frame; an address field (A), a control field (C), a facsimile control field (FCF), a facsimile information field (FIF), and a frame check sequence (FCS).
It is to be noted herein that a preamble, which is shown in FIG. 9 and constituted by flag patterns is added inevitably to a head of each control signal for about one second. For instance, a delivery time of CFR frame is about 0.2 second, and about one second of preamble is added thereto. In a series of facsimile transmission control procedure sequence, a time expended for these preamble transmissions is about six seconds in, for example, a sequence shown in FIG. 8, because six preambles are added to eight procedure signals (see FIG. 9). A reduction of such expended time is also a problem in view of a total reduction for a time of transmission control procedure.
In the case when a procedure based on the Recommendation T.3 is employed for a facsimile transmission control procedure for relay purpose through a public telephone network in such event where a facsimile communication is relayed between mobile communication control units each of which involves a radio link, and they are disposed at different places from one another, there is such a problem as described hereinafter. The problem is in that there is a possibility of breakdown of communication by transmission delay due to redelivery control of data in the case when data error arises on a radio link during a transmission and a response thereof in mobile communication control units each including such radio link. Such problem mentioned herein will be described.
FIG. 10 is a diagram showing such a situation that CFR transmitted by a receive terminal 3 is relayed with a time delay to a transmit terminal 1 as a result of an influence of transmission delay in a phase B of a facsimile transmission control procedure (see FIG. 8), so that a starting time of a phase C deviates remarkably in the transmit terminal 1 and the receive terminal 3.
FIG. 11 is a diagram showing such a situation that a time condition defined by the Recommendation T.30 comes to be not satisfied finally in a phase D of the transmit terminal 1 because of the deviation of starting time of the phase C in the transmit terminal 1 and the receive terminal 3 shown in FIG. 10. FIG. 12 is a diagram for explaining a problem involved in a time length of procedure signal defined by the Recommendation T.30.
As shown in FIG. 10, there may be such a case where a first time redelivery of NSS, TSI, DCS, and TCF has been executed because of an elapse of time for a period of T4 (three seconds) after transmission of TCF in a mobile communication control unit 8 at such a time that the NSS, TSI, and DCS, which are transmitted from a transmit terminal 1, reach a receive terminal 3 with a delay time involved, and CFR is transmitted after receiving these signals by the receive terminal 3. In this case, a mobile communication control unit 9 is in a state of reception for these signals from the mobile communication control unit 8, so that the mobile communication control unit 9 cannot transmit the CFR received from the receive terminal 3 to the mobile communication control unit 8. The mobile communication control unit 9 transmits the CFR at the time when reception of these signals was completed from the mobile communication control unit 8, and a mobile terminal 5 intends to transmit CFR to a transmit terminal 1.
At which time, there may be a case where the transmit terminal 1 is in the process of a second time redelivery of NSS, TSI, DCS, and TCF. In this case, the mobile terminal 5 transmits CFR to the transmit terminal 1 after receiving the second time redelivery of these signals from the transmit terminal 1. The transmit terminal 1 transmits an image signal after receiving the CFR. At this occasion, there may be a case where the mobile communication control unit 8 is in making the second time redelivery of NSS, TSI, DCS, and TCF with respect to the mobile communication control unit 9. This is because a communication is stopped in the mobile communication control unit 9 in the case when no image signal is received thereby within a period of time T2 after transmitting CFR. In order to avoid such stop of communication, these signals are to be redelivered.
In case of an example of FIG. 10, NSS, TSI, DCS, and TCF are redelivered from the mobile communication control unit 8 in the case where a preparation for transmitting an image signal is not completed within four seconds after receiving CFR from the mobile communication control unit 9. Accordingly, although the mobile communication control unit 8 has received an image signal from the mobile terminal 5 at this occasion, the mobile communication control unit 8 is also in making the second redelivery of these signals, so that the image signal is transmitted to the mobile communication control unit 9 after completing the redelivery.
When a preparation for delivering an image signal is not completed within four seconds after receiving CFR from the receive terminal 3 in also a mobile terminal 7, NSS, TSI, DCS, and TCF are redelivered to the receive terminal 3. Accordingly, there may be such a case where the second delivery of these signals is made at the time when the image signal is received from the mobile communication control unit 9. In this case also, the mobile terminal 7 starts to transmit the image signal to the receive terminal 3 after completing the redelivery of these signals.
As described above, there is such a possibility that a remarkable time deviation arises at a starting time of a phase C in the transmit terminal 1 and the receive terminal 3. How much degree of time delay arises is dependent upon a degree of delay due to a processing time at each node on a transmission path, or that of a delay due to redelivery control derived from data error on a radio link. However, it has been confirmed that it is about four seconds from a transmission of an image signal by the transmit terminal 1 until the mobile communication control-unit 8 starts to transmit the image signal, and it is about eight seconds until the receive terminal 3 starts to receive the image signal in accordance with a situation of actual measurement (without accompanying any appearance of data error on a radio link).
In the following, a situation wherein a time for transmission control procedure in a phase D is affected by a deviation of starting time of a phase C will be described by referring to FIG. 11. When transmission of an image signal is completed, the transmit terminal 1 transmits EOP, while the mobile communication control unit 8 is yet in the process of transmitting the image signal because of a delay in starting the phase C described relating to FIG. 10. After a lapse of a certain period of time corresponding to that of the delay in starting the phase C, the mobile communication control unit 8 completes transmission of the image signal, and transmits EOP.
The receive terminal 3 transmits MCF after receiving the image signal and EOP following thereto. When the MCF is received by the mobile communication control unit 8, it transmits the signal to a side of the transmit terminal 1. At this occasion, however, there may be such a case where the transmit terminal 1 completes a second time redelivery of EOP, i.e., a third time transmission of EOP. In this case, a response of MCF is in time by only a narrow margin with respect to the third time EOP, which is transmitted from the transmit terminal 1. Under the situation as described above, when a transmission delay due to redelivery control derived from data error on a radio link is added, the MCF response is not in time with respect to the third time EOP, which is transmitted from the transmit terminal 1, so that there is a high possibility of appearing an abnormal communication.
Referring to FIG. 12, a procedure signal defined by the Recommendation T.30 involves a margin of about one second. Thus, there appears a transmission delay, because the procedure signal is received at each node on a transmission path, and the signal is relayed successively to the following section. Due to this transmission delay, since a redelivery sequence for the procedure signal is conducted, a further time delay appears, whereby the communication may be failed. Such an occasion mentioned herein will be described in conjunction with FIG. 12 wherein MPS and MCF are used as procedure signals, and an explanation will be made relating thereto.
As shown in FIG. 12, when the transmit terminal 1 transmits MPS, a mobile terminal 5 transmits the MPS after receiving the same to the mobile communication control unit 8. Since the MPS has a margin of time for about one second, when the signal is relayed by the mobile terminal 5, a delay of about one second appears. Such situation is found also in the mobile communication control units 8 and 9 as well as in the mobile terminal 7.
When this MPS is received by the receive terminal 3 with a transmission delay on a radio link, the receive terminal 3 returns MCF as a response to the reception of the MPS. In this case, however, the mobile communication control unit 8 cannot obtain the response within a period of time T4 after transmitting MPS to the mobile communication control unit 9. Accordingly, there may be a case where the mobile communication control unit 8 redelivers MPS. For this reason, the mobile communication control unit 9 cannot transmit MCF in spite of the fact that the MCF has been received from a side of the receive terminal 3, so that the mobile communication control unit 9 transmits the MCF after completing reception of MPS. The mobile communication control unit 8 receives MCF after completing a redelivery of the MPS, and transmits the MCF to the mobile terminal 5. At this occasion, there may be such a case that the transmit terminal 1 is in the process of a second time redelivery of MPS, i.e., a third time transmission thereof. Under the circumstances, the mobile terminal 5 returns MCF that is a response to the third time transmission of MPS.
Under the situation as described above, when a transmission delay due to redelivery control derived from data error on a radio link is added, the MCF response is not in time with respect to the third time MPS, which is transmitted from the transmit terminal 1, so that there is a high possibility of appearing an abnormal communication.